In systems such as High Speed Uplink Packet Access (HSUPA) for UMTS (Universal Mobile Telecommunication System), mobile stations (MSs) transmit a control signal in parallel with a discontinuous data signal, the control signal continuing to be transmitted during the periods when the data signal is not transmitted.
The transmit power level of the control signal may be reduced during periods when data transmission is discontinued, which conserves power and reduces the average interference generated by the transmissions, and increased during at least some of each transmission period of the data signal to enable reliable reception of the control signal.
Systems such as UMTS use closed loop power control, whereby an MS's transmit power is adjusted by transmit power control (TPC) commands received from one or more base stations (BSs). The power control commands are typically generated at the BS(s) by means of comparing with a target level the Signal-to-Interference Ratio (SIR) of the control signal received from the UE, and generating a “down” command if the SIR is above the target level or an “up” command if the SIR is below the target level. Consequently, if it is desired to apply an increase to the transmit power level of the control signal when a data signal is transmitted, it is necessary also to raise the target SIR level in order to avoid any power step applied at the MS being cancelled out after a short period of time by the response to power control commands generated by the BS(s) using the previous SIR target level.
Ideally, an adjustment to the SIR target would be applied at the BS synchronously with each of the changes of control signal transmit power described above. This may be achievable if the BS can quickly detect when the change in control signal transmit power has been applied, for example by detecting the start of data transmission.
However, in practice, it may be difficult for the BS to detect when the change in control signal transmit power has been applied before the closed loop power control has cancelled out the change in control signal transmit power. For example, in UMTS the power control commands are transmitted every timeslot (0.666 ms). If, for example, a 3 dB change is applied to the control signal transmit power, and a 1 dB step is used for the inner loop power control, then the inner loop power control would have fully counteracted the change in the control signal transmit power within 3 timeslots. If the BS is to avoid any counteracting by the inner loop power control, it has less than 1 timeslot in which to detect the application of the increase in control signal transmit power.